The present invention relates to the field of image display systems, and more particularly, to methods and systems for reducing contouring artifacts in image display systems.
All conventional digital video signals are quantized during various video-processing steps. For example, analog to digital conversion and certain compression techniques involve quantization. One drawback of quantization is that quantization tends to cause a visual artifact known as contouring in regions of the picture with very low intensity gradients. Contouring occurs when the quantization of an image signal causes contours to appear in an output image that do not exist in the input image. More specifically, when an input signal is quantized a smooth image gradient may be transformed into several large blocks of adjacent pixels, wherein each pixel in a block is assigned an identical image signal value. If these large blocks of adjacent pixels are not separated by a region of non-homogenous pixels, the blocks will cause a xe2x80x9cstair stepxe2x80x9d effect and the smooth curve of the original image will appear to be a series of single-color flat surfaces. Contouring is associated with the ability of the human visual system to perceive small changes in image intensity in areas of an image that have low spatial variation in image intensity. If an insufficient number of bits is used to represent intensities in such areas, the human visual system perceives the change in intensity as happening in steps and not in a continuous manner.
The present invention is directed to overcoming this drawback.
Briefly stated, the present invention is directed towards detecting contouring artifacts in a received video signal and removing the detected artifacts by dithering, by adding least significant bits to selected pixels in the video signal, or by utilizing unused states.